Automated color tuning of an led based illumination device

ABSTRACT

The color of light emitted by an assembled light emitting diode (LED) based illumination device with at least two different wavelength converting materials is automatically tuned to within a predefined tolerance of a target color point by modifying portions of the wavelength converting materials. The color of light emitted from the assembled LED based illumination device is measured and a material modification plan is determined based at least in part on the measured color of light and a desired color of light to be emitted. The material modification plan may further include the location of the wavelength converting materials to be modified. The wavelength converting materials are selectively modified in accordance with the material modification plan so that the assembled LED based illumination device emits a second color of light that is within a predetermined tolerance of a target color point.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC §119 to U.S. ProvisionalApplication No. 61/738,314, filed Dec. 17, 2012, which is incorporatedby reference herein in its entirety.

TECHNICAL FIELD

The described embodiments relate to illumination devices that includeLight Emitting Diodes (LEDs).

BACKGROUND

The use of light emitting diodes in general lighting is still limiteddue to limitations in light output level or flux generated by theillumination devices. Illumination devices that use LEDs also typicallysuffer from poor color quality characterized by color point instability.The color point instability varies over time as well as from part topart. Poor color quality is also characterized by poor color rendering,which is due to the spectrum produced by the LED light sources havingbands with no or little power. Further, illumination devices that useLEDs typically have spatial and/or angular variations in the color.Additionally, illumination devices that use LEDs are expensive due to,among other things, the necessity of required color control electronicsand/or sensors to maintain the color point of the light source or usingonly a small selection of produced LEDs that meet the color and/or fluxrequirements for the application.

Consequently, improvements to illumination device that uses lightemitting diodes as the light source are desired.

SUMMARY

The color of light emitted by an assembled light emitting diode (LED)based illumination device with at least two different wavelengthconverting materials is automatically tuned to within a predefinedtolerance of a target color point by modifying portions of thewavelength converting materials. The color of light emitted from theassembled LED based illumination device is measured and a materialmodification plan is determined based at least in part on the measuredcolor of light and a desired color of light to be emitted. The materialmodification plan may further include the location of the wavelengthconverting materials to be modified, which may be based on, e.g., theoutput beam intensity distribution, color conversion efficiency, a coloruniformity, and a temperature distribution over a light emittingsurface. The wavelength converting materials are selectively modified inaccordance with the material modification plan so that the assembled LEDbased illumination device emits a second color of light that is within apredetermined tolerance of a target color point. For example, thewavelength converting materials may be selectively modified by removingamounts the wavelength converting materials by laser ablation,mechanical scribing, ion etching, chemical etching, electrical dischargemachining, plasma etching, and chemical mechanical polishing or addingamounts of wavelength converting materials by jet dispensing, spraycoating, screen printing, and blade coating.

Further details and embodiments and techniques are described in thedetailed description below. This summary does not define the invention.The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2, and 3 illustrate three exemplary luminaires, including anillumination device, reflector, and light fixture.

FIG. 4 illustrates a perspective cut-away view of components in anembodiment of an LED based illumination device including a basereflector structure that physically couples a transmissive plate and anLED mounting board.

FIG. 5 illustrates a perspective cut-away view of components in anotherembodiment of an LED based illumination device including a basereflector structure that physically couples a transmissive plate and anLED mounting board.

FIG. 6 illustrates a perspective cut-away view of components in anotherembodiment of an LED based illumination device including a basereflector structure that physically couples a transmissive plate and anLED mounting board.

FIG. 7 illustrates a side view of components in another embodiment of anLED based illumination device with a total internal reflection (TIR)lens structure to direct light emitted from LEDs to a transmissiveplate.

FIG. 8 illustrates a side view of components in another embodiment of anLED based illumination device with a dam of reflective materialsurrounding the LEDs and supporting a transmissive plate.

FIG. 9 illustrates a side view of components in another embodiment of anLED based illumination device with a shaped lens disposed over the LEDsand thermally coupled to the LED mounting board.

FIG. 10 illustrates a side view of components in another embodiment ofan LED based illumination device with multiple transmissive plates.

FIG. 11 illustrates a side view of components in another embodiment ofan LED based illumination device with droplets of a wavelengthconverting material uniformly applied to the surface of transmissivelayer.

FIG. 12 illustrates a side view of components in another embodiment ofan LED based illumination device with droplets of a wavelengthconverting material applied to the surface of transmissive layer in anon-uniform pattern.

FIG. 13 illustrates a side view of components in another embodiment ofan LED based illumination device with droplets of different wavelengthconverting materials applied to the surface of transmissive layer in anon-uniform pattern.

FIG. 14 is illustrative of a system for automatically tuning the colorof light emitted from an assembled LED based illumination device withina predefined tolerance of a target color point by removing portions oftwo different wavelength converting materials.

FIG. 15 is illustrative of another embodiment of a system forautomatically tuning the color of light emitted from an assembled LEDbased illumination device within a predefined tolerance of a targetcolor point by removing portions of two different wavelength convertingmaterials.

FIG. 16 illustrates a method of automatically tuning the color of lightemitted from an assembled LED based illumination device within apredefined tolerance of a target color point by modifying portions of atleast two different wavelength converting materials.

FIG. 17 is illustrative of a (xy) chromaticity diagram based on the CIE1931 XYZ color space.

FIG. 18 is a diagram illustrating color points of LED devices andpredetermined target color points on the black-body curve from the CIE1960 UCS diagram where the horizontal axis represents CCT and thevertical axis represents the degree of departure (Δuv) from theblack-body curve.

FIG. 19 is illustrative of an image collected by a camera of the lightemitting surface of LED based illumination device.

FIG. 20 is a plot illustrative of the luminance across the emittingsurface of LED based illumination device at Section line A illustratedin FIG. 19.

FIG. 21 shows an illustrative plotline indicative of a spatial variationin intensity of light emitted from LEDs in a plane that is coplanar withtransmissive plate.

FIG. 22 is illustrative of a material modification plan that includestrajectories of material removal that are a fixed distance from eachunderlying LED location.

DETAILED DESCRIPTION

Reference will now be made in detail to background examples and someembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

FIGS. 1, 2, and 3 illustrate three exemplary luminaires, labeled 150,150′, and 150″, which are sometimes collectively referred to asluminaire 150. The luminaire illustrated in FIG. 1 includes an LED basedillumination device 100 with a rectangular form factor. The luminaireillustrated in FIG. 2 includes an LED based illumination device 100′with a circular form factor. The luminaire illustrated in FIG. 3includes an LED based illumination device 100′ integrated into aretrofit lamp device. These examples are for illustrative purposes.Examples of LED based illumination devices of general polygonal andelliptical shapes may also be contemplated, and in general, LED basedillumination devices 100 and 100′ may be collectively referred to as LEDbased illumination device 100. As illustrated in FIG. 1, luminaire 150includes illumination device 100, reflector 125, and light fixture 120.FIG. 2 shows luminaire 150′ with illumination device 100′, reflector125′, and light fixture 120′ and FIG. 3 shows luminaire 150″ withillumination device 100′, reflector 125″, and light fixture 120″.Reflectors 125, 125′, and 125″ are sometimes collectively referred toherein as reflector 125, and light fixtures 120, 120′, and 120″ aresometimes collectively referred to herein as light fixture 120. Asdepicted, light fixture 120 includes a heat sink capability, andtherefore may be sometimes referred to as heat sink 120. However, lightfixture 120 may include other structural and decorative elements (notshown). Reflector 125 is mounted to illumination device 100 to collimateor deflect light emitted from illumination device 100. The reflector 125may be made from a thermally conductive material, such as a materialthat includes aluminum or copper and may be thermally coupled toillumination device 100. Heat flows by conduction through illuminationdevice 100 and the thermally conductive reflector 125. Heat also flowsvia thermal convection over the reflector 125. Reflector 125 may be acompound parabolic concentrator, where the concentrator is constructedof or coated with a highly reflecting material. Optical elements, suchas a diffuser or reflector 125 may be removably coupled to illuminationdevice 100, e.g., by means of threads, a clamp, a twist-lock mechanism,or other appropriate arrangement. As illustrated in FIG. 3, a reflector125 may include sidewalls 126 and a window 127 that are optionallycoated, e.g., with a wavelength converting material, diffusing materialor any other desired material.

As depicted in FIGS. 1, 2, and 3, illumination device 100 is mounted toheat sink 120. Heat sink 120 may be made from a thermally conductivematerial, such as a material that includes aluminum or copper and may bethermally coupled to illumination device 100. Heat flows by conductionthrough illumination device 100 and the thermally conductive heat sink120. Heat also flows via thermal convection over heat sink 120.Illumination device 100 may be attached to heat sink 120 by way of screwthreads to clamp the illumination device 100 to the heat sink 120. Tofacilitate easy removal and replacement of illumination device 100,illumination device 100 may be removably coupled to heat sink 120, e.g.,by means of a clamp mechanism, a twist-lock mechanism, or otherappropriate arrangement. Illumination device 100 includes at least onethermally conductive surface that is thermally coupled to heat sink 120,e.g., directly or using thermal grease, thermal tape, thermal pads, orthermal epoxy. For adequate cooling of the LEDs, a thermal contact areaof at least 50 square millimeters, but preferably 100 square millimetersshould be used per one watt of electrical energy flow into the LEDs onthe board. For example, in the case when 20 LEDs are used, a 1000 to2000 square millimeter heat sink contact area should be used. Using alarger heat sink 120 may permit the LEDs 102 to be driven at higherpower, and also allows for different heat sink designs. For example,some designs may exhibit a cooling capacity that is less dependent onthe orientation of the heat sink. In addition, fans or other solutionsfor forced cooling may be used to remove the heat from the device. Thebottom heat sink may include an aperture so that electrical connectionscan be made to the illumination device 100.

FIGS. 4, 5, and 6 illustrate perspective cut-away views of components ofvarious embodiments of LED based illumination device 100. It should beunderstood that as defined herein an LED based illumination device isnot an LED, but is an LED light source or fixture or component part ofan LED light source or fixture. For example, an LED based illuminationdevice may be an LED based replacement lamp such as depicted in FIG. 3.LED based illumination device 100 includes one or more LED die orpackaged LEDs and a mounting board to which LED die or packaged LEDs areattached. In one embodiment, the LEDs 102A and 102B, sometimes referredto herein as LEDs 102 are are packaged LEDs, such as the Luxeon Rebelmanufactured by Philips Lumileds Lighting. Other types of packaged LEDsmay also be used, such as those manufactured by OSRAM (Oslon package),Luminus Devices (USA), Cree (USA), Nichia (Japan), or Tridonic(Austria). As defined herein, a packaged LED is an assembly of one ormore LED die that contains electrical connections, such as wire bondconnections or stud bumps, and possibly includes an optical element andthermal, mechanical, and electrical interfaces. The LED chip typicallyhas a size about 1 mm by 1 mm by 0.5 mm, but these dimensions may vary.In some embodiments, the LEDs 102 may include multiple chips. Themultiple chips can emit light of similar or different colors, e.g., red,green, and blue. LEDs 102 are mounted to mounting board 104. The lightemitted from LEDs 102 is directed to transmissive plate 174. A thermallyconductive base reflector structure 171 promotes heat dissipation fromthe transmissive plate 174 to the mounting board 104, upon which theLEDs 102 are mounted.

FIG. 5 illustrates LED based illumination device 100 with the basereflector structure 171′. As illustrated, the base reflector structure171′ includes deep reflector surfaces 171B that direct light emittedfrom LEDs 102 to transmissive plate 174. In addition, base reflectorstructure 171′ includes a centrally located feature 171C that thermallyconnects transmissive plate 174 and mounting board 104. As illustrated,base reflector structure 171′ is constructed from one part to minimizemanufacturing complexity.

As illustrated in FIG. 6, base reflector structure 171″ includes athermally conductive insert 171D that thermally couples transmissiveplate 174 and mounting board 104. In this manner, base reflectorstructure may be constructed from a low cost material (e.g., plastic)and the thermally conductive insert 171D may be constructed from amaterial optimized for thermal conductivity (e.g., aluminum or copper).

As depicted in FIGS. 4-6, base reflector structure 171 is in physicalcontact with transmissive plate 174 and mounting board 104. However, insome other embodiments, base reflector structure 171 may be in physicalcontact with transmissive plate 174 and heat sink 120. In this manner, amore direct thermal path between transmissive plate 174 and heat sink120 is realized. In one example, elements of base reflector structure171 may be configured to pass through voids in LED board 104 to directlycouple transmissive plate 174 to heat sink 120.

Base reflector structure 171 may have a high thermal conductivity tominimize thermal resistance. By way of example, base reflector structure171 may be made with a highly thermally conductive material, such as analuminum based material that is processed to make the material highlyreflective and durable. By way of example, a material referred to asMiro®, manufactured by Alanod, a German company, may be used.

FIG. 7 is illustrative of another configuration of LED basedillumination device 100, which is similar to that shown in FIGS. 4-6,like designated elements being the same. As illustrated in FIG. 7, LEDbased illumination device 100 may include a total internal reflection(TIR) lens structure 178 to direct light emitted from LEDs 102 totransmissive plate 174.

FIG. 8 is illustrative of another configuration of LED basedillumination device 100, which is similar to that shown in FIGS. 4-6,like designated elements being the same. As illustrated in FIG. 8, LEDbased illumination device 100 includes a number of LEDs 102A-F,collectively referred to as LEDs 102, arranged in a chip on board (COB)configuration. LED based illumination device 100 also includes a basereflector structure including a reflective material 176 disposed in thespaces between each LED and a dam of reflective material 175 thatsurrounds the LEDs 102 and supports transmissive plate 174. In someexamples, reflective materials 175 and 176 are a white, reflectivesilicone-based material. In the embodiment depicted in FIG. 8, the spacebetween LEDs 102 and transmissive plate 174 is filled with anencapsulating optically translucent material 177 (e.g., silicone) topromote light extraction from LEDs 102 and to separate LEDs 102 from theenvironment.

FIG. 9 is illustrative of another configuration of LED basedillumination device 100, which is similar to that shown in FIGS. 4-6,like designated elements being the same. As illustrated, LED basedillumination device 100 includes a shaped lens 172 disposed over LEDs102A, 102B, and 102C, collectively referred to as LEDs 102. Asillustrated, shaped lens 172 includes at least one wavelength convertingmaterial at the light emitting surface of shaped lens 172. Shaped lens172 is directly coupled to mounting board 104 to promote heat flow fromshaped lens 172 to mounting board 104. In this manner, heat generated bycolor conversion on surfaces of shaped lens 172 is efficientlytransferred to mounting board 104 and removed from LED basedillumination device 100 via heat sink 120. In some other embodiments,shaped lens 172 is directly coupled to heat sink 120.

The optical surfaces of base reflector structure 171 may be treated toachieve high reflectivity. For example the optical surface of basereflector structure 171 may be polished, or covered by one or morereflective coatings (e.g., reflective materials such as Vikuiti™ ESR, assold by 3M (USA), Lumirror™ E60L manufactured by Toray (Japan), ormicrocrystalline polyethylene terephthalate (MCPET) such as thatmanufactured by Furukawa Electric Co. Ltd. (Japan), apolytetrafluoroethylene PTFE material such as that manufactured by W.L.Gore (USA) and Berghof (Germany)). Also, highly diffuse reflectivecoatings can be applied to optical surfaces of base reflector structure171. Such coatings may include titanium dioxide (TiO2), zinc oxide(ZnO), and barium sulfate (BaSO4) particles, or a combination of thesematerials.

In some embodiments, base reflector structure 171 may be constructedfrom or include a reflective, ceramic material, such as ceramic materialproduced by CerFlex International (The Netherlands). In someembodiments, portions of any of the optical surfaces of base reflectorstructure 171 may be coated with a wavelength converting material.

LEDs 102 can emit different or the same colors, either by directemission or by phosphor conversion, e.g., where phosphor layers areapplied to the LEDs as part of the LED package. The illumination device100 may use any combination of colored LEDs 102, such as red, green,blue, amber, or cyan, or the LEDs 102 may all produce the same colorlight. Some or all of the LEDs 102 may produce white light. In addition,the LEDs 102 may emit polarized light or non-polarized light and LEDbased illumination device 100 may use any combination of polarized ornon-polarized LEDs. In some embodiments, LEDs 102 emit either blue or UVlight because of the efficiency of LEDs emitting in these wavelengthranges. The light emitted from the illumination device 100 has a desiredcolor when LEDs 102 are used in combination with wavelength convertingmaterials on transmissive plate 174 or shaped lens 172, for example. Bytuning the chemical and/or physical (such as thickness andconcentration) properties of the wavelength converting materials and thegeometric properties of the coatings on the surfaces of transmissiveplate 174 or shaped lens 172, specific color properties of light outputby LED based illumination device 100 may be specified, e.g., colorpoint, color temperature, and color rendering index (CRI).

For purposes of this patent document, a wavelength converting materialis any single chemical compound or mixture of different chemicalcompounds that performs a color conversion function, e.g., absorbs anamount of light of one peak wavelength, and in response, emits an amountof light at another peak wavelength.

In some examples, a wavelength converting material is a phosphor ormixture of different phosphors. By way of example, phosphors may bechosen from the set denoted by the following chemical formulas:Y3Al5O12:Ce, (also known as YAG:Ce, or simply YAG) (Y,Gd)3Al5O12:Ce,CaS:Eu, SrS:Eu, SrGa2S4:Eu, Ca3(Sc,Mg)2Si3O12:Ce, Ca3Sc2Si3O12:Ce,Ca3Sc2O4:Ce, Ba3Si6O12N2:Eu, (Sr,Ca)AlSiN3:Eu, CaAlSiN3:Eu,CaAlSi(ON)3:Eu, Ba2SiO4:Eu, Sr2SiO4:Eu, Ca2SiO4:Eu, CaSc2O4:Ce,CaSi2O2N2:Eu, SrSi2O2N2:Eu, BaSi2O2N2:Eu, Ca5(PO4)3Cl:Eu,Ba5(PO4)3Cl:Eu, Cs2CaP2O7, Cs2SrP2O7, Lu3Al5O12:Ce, Ca8Mg(SiO4)4Cl2:Eu,Sr8Mg(SiO4)4Cl2:Eu, La3Si6N11:Ce, Y3Ga5O12:Ce, Gd3Ga5O12:Ce,Tb3Al5O12:Ce, Tb3Ga5O12:Ce, and Lu3Ga5O12:Ce.

In one example, the adjustment of color point of the illumination devicemay be accomplished by adding or removing wavelength converting materialfrom transmissive plate 174 or shaped lens 172, which similarly may becoated or impregnated with one or more wavelength converting materials.In one embodiment a red emitting phosphor 181 such as an alkaline earthoxy silicon nitride covers a portion of transmissive plate 174 or shapedlens 172, and a yellow emitting phosphor 180 such as YAG covers anotherportion of transmissive plate 174 or shaped lens 172, as illustrated inFIGS. 4-9.

In some embodiments, the phosphors are mixed in a suitable solventmedium with a binder and, optionally, a surfactant and a plasticizer.The resulting mixture is deposited by any of spraying, screen printing,blade coating, jetting, or other suitable means. By choosing the shapeand height of the transmissive plate 174 or shaped lens 172, andselecting which portions of transmissive plate 174 or shaped lens 172will be covered with a particular phosphor or not, and by optimizationof the layer thickness and concentration of a phosphor layer on thesurfaces, the color point of the light emitted from the device can betuned as desired.

In one example, a single type of wavelength converting material may bepatterned on a portion of transmissive plate 174 or shaped lens 172. Byway of example, a red emitting phosphor 181 may be patterned ondifferent areas of the transmissive plate 174 or shaped lens 172 and ayellow emitting phosphor 180 may be patterned on other areas oftransmissive plate 174 or shaped lens 172. In some examples, the areasmay be physically separated from one another. In some other examples,the areas may be adjacent to one another. The coverage and/orconcentrations of the phosphors may be varied to produce different colortemperatures. It should be understood that the coverage area of the redand/or the concentrations of the red and yellow phosphors will need tovary to produce the desired color temperatures if the light produced bythe LEDs 102 varies. The color performance of the LEDs 102, red phosphorand the yellow phosphor may be measured and modified by any of adding orremoving phosphor material based on performance so that the finalassembled product produces the desired color temperature.

Transmissive plate 174 and shaped lens 172 may be constructed from asuitable optically transmissive material (e.g., sapphire, alumina, crownglass, polycarbonate, and other plastics).

In some embodiments, multiple, stacked transmissive layers are employed.Each transmissive layer includes different wavelength convertingmaterials. For example, as illustrated in FIG. 10, transmissive layer174 includes wavelength converting material 180 over the surface area oftransmissive layer 174. In addition, a second transmissive layer 163 isplaced over and in contact with transmissive layer 174. Transmissivelayer 163 includes wavelength converting material 181. Although, asillustrated in FIG. 10, transmissive layer 163 is placed over and incontact with transmissive layer 174, a space may be maintained betweenthe two elements. This may be desirable to promote cooling of thetransmissive layers. For example, airflow may by introduced through thespace to cool the transmissive layers.

In some embodiments, any of the wavelength converting materials may beapplied as a pattern (e.g., stripes, dots, blocks, droplets, etc.). Forexample, as illustrated in FIG. 11, droplets of wavelength convertingmaterial 180 are uniformly applied to the surface of transmissive layer174. Shaped droplets may improve extraction efficiency by increasing theamount of surface area of the droplet.

As illustrated in FIG. 12, in some embodiments, droplets of wavelengthconverting material 180 may be spaced on transmissive layer 174 in anon-uniform pattern. For example, a group of droplets 165 located overLED 102C is densely packed (e.g., droplets in contact with adjacentdroplets), while a group of droplets 164 located over a space betweenLEDs 102A and 102B is loosely packed (e.g., droplets spaced apart fromadjacent droplets). In this manner, the color point of light emittedfrom LED based illumination device 100 may be tuned by varying thepacking density of droplets on transmissive layer 174.

As illustrated in FIG. 13, in some embodiments, droplets of differentwavelength converting materials may be placed in different locations oftransmissive layer 174 and may also be placed in a non-uniform pattern.For example, group of droplets 164 may include wavelength convertingmaterial 180 and group of droplets 165 may include a combination ofdroplets including wavelength converting material 181 and wavelengthconverting material 182. In this manner, combinations of differentwavelength converting materials are located relative to LEDs 102 invarying densities to achieve a desired color point of light emitted fromLED based illumination device 100.

In the illustrated embodiments, wavelength converting materials arelocated on the surface of transmissive layer 174. However, in some otherembodiments, any of the wavelength converting materials may be embeddedwithin transmissive layer 174, on the side of transmissive layer 174facing LEDs 102, or any combination thereof.

The area between LEDs 102 and transmissive plate 174 or shaped lens 172may be filled with a non-solid material, such as air or an inert gas, sothat the LEDs 102 emit light into the non-solid material. By way ofexample, the cavity may be hermetically sealed and Argon gas used tofill the cavity. Alternatively, Nitrogen may be used. In otherembodiments, the area between LEDs 102 and transmissive plate 174 orshaped lens 172 may be filled with a solid encapsulate material. By wayof example, silicone may be used to fill the cavity. In some otherembodiments, color conversion cavity 160 may be filled with a fluid topromote heat extraction from LEDs 102. In some embodiments, wavelengthconverting material may be included in the fluid to achieve colorconversion.

With two or more of wavelength converting materials, each with differentwavelength converting properties, the LED based illumination device 100may produce a predetermined or target color point with a high degree ofaccuracy.

FIG. 17 is illustrative of a (xy) chromaticity diagram based on the CIE1931 XYZ color space. The CIE 1931 color space is based on three colormatching functions. The three tristimulus values express the CIE 1931XYZ color space as a three dimensional color space. Each color matchingfunction relates a given spectrum, S(λ), to each of the threetristimulus values, X, Y, and Z, as described in equation (1).

X ₁₉₃₁ =∫CMF _(X) S(λ)dλ

Y ₁₉₃₁ =∫CMF _(Y) S(λ)dλ

Z ₁₉₃₁ =∫CMF _(Z) S(λ)dλ  (1)

The xy chromaticity diagram of FIG. 17 is a projection of the threedimensional CIE 1931 XYZ color space onto a two dimensional space (xy)such that brightness is ignored. Each color coordinate (x,y) may beexpressed as a function of the three tristimulus values as described inequation (2).

$\begin{matrix}{{x = \frac{X}{X + Y + Z}}{y = \frac{Y}{X + Y + Z}}} & (2)\end{matrix}$

There are other color spaces that are simple projective transformationsof the CIE 1931 XYZ color space. For example, both the CIE 1960 uniformcolor scale (CIE 1960 UCS) and the CIE 1976 uniform color scale (CIE1976 UCS) are simple transformations of the CIE 1931 XYZ color space.The CIE 1960 UCS expresses two dimensional chromaticity (uv) as afunction of the three tristimulus values as described in equation (3).

$\begin{matrix}{{u = \frac{4X}{X + {15Y} + {3Z}}}{v = \frac{6Y}{X + {15Y} + {3Z}}}} & (3)\end{matrix}$

The CIE 1976 UCS expresses two dimensional chromaticity (u′v′) as afunction of the three tristimulus values as described in equation (4).

$\begin{matrix}{{u^{\prime} = \frac{4X}{X + {15Y} + {3Z}}}{v^{\prime} = \frac{9Y}{X + {15Y} + {3Z}}}} & (4)\end{matrix}$

The CIE 1960 UCS color space has generally been superseded by the CIE1976 UCS color space as an expression of uniform chromaticity. However,the CIE 1960 UCS color space is still useful as an expression ofchromaticity because the isothermal lines of correlated colortemperature (CCT) are aligned perpendicular to the Planckian locus inCIE 1960 UCS. In the context of the CIE 1960 UCS, the degree ofdeparture is the distance between the color point of the light producedby the light source and the Planckian locus along a line of constantCCT. The degree of departure is referred to in units of Δuv in CIE 1960UCS. Thus, the color point of a white light source may be described as aCCT value and a Δuv value, i.e., the degree of departure from theblack-body curve as measured in the CIE 1960 color space. It followsthat the specification for color of light output by LED basedillumination device 100 can be expressed as a CCT value within apredetermined tolerance and a Δuv value within a predeterminedtolerance. FIG. 18 illustrates a plot of the black-body curve 400,sometimes referred to as a Planckian locus, parallel to the horizontalaxis and units of Δuv along the vertical axis in the context of the CIE1960 chromaticity diagram. Target color points 256-258 are illustratedas exemplary target color points. The degree of departure from thetarget color point is referred to in units of Δuv. When the color pointof a light source varies significantly from a predetermined target colorpoint, the color of the light will be perceptively different from thedesired color. Moreover, when light sources are near each other, e.g.,in accent lighting or a display, even slight color differences arenoticeable and considered undesirable.

Producing light sources that generate light near a target color point isdesirable. For example, when used for purposes of general illumination,it is desirable that the LED based illumination device 100 produce whitelight with a particular correlated color temperature (CCT). CCT relatesto the temperature of a black-body radiator and temperatures between2700K and 6000K are typically useful for general illumination purposes.Higher color temperatures are considered “cool” as they are bluish incolor, while lower temperatures are considered “warm” as they containmore yellow-red colors. By way of example, CCTs of 2700K, 3000K, 3500K,4000K, 4200K, 5000K, 6500K are often desirable. In another example,light emitted from an LED based illumination device targeting any of CIEilluminant series A, B, C, D, E, and F are desirable.

As illustrated in FIG. 17, the chromaticity of a black-body radiator inCIE 1931 color space is represented by curve 200. This curve issometimes referred to as the Planckian locus. Ideally, light sourcesproduce light that lies on the black-body curve 200 at a target colorpoint. In practice, however, producing light at a target color point onthe black-body curve 200 is difficult, particularly with an LED lightsource because of the lack of precise control over the light output ofan LED light source manufactured using current processes. Typically,there will be some distance between the color point of the lightproduced by the light source and the target color point on theblack-body curve 200, which is known as the degree of departure from thetarget color point on the black-body curve.

An LED is typically binned after a production run based on a variety ofcharacteristics derived from its spectral power distribution. The costof the LEDs is determined by the size (distribution) of the bin. Forexample, a particular LED may be binned based on the value of its peakwavelength. The peak wavelength of an LED is the wavelength where themagnitude of its spectral power distribution is maximal. Peak wavelengthis a common metric to characterize the color aspect of the spectralpower distribution of blue LEDs. Many other metrics are commonly used tobin LEDs based on their spectral power distribution (e.g. dominantwavelength, xy color point, uv color point, etc.). It is common for blueLEDs to be separated for sale into bins with a range of peak wavelengthof five nanometers.

As discussed above, LED based illumination device 100 includes a board104 with a plurality of LEDs 102. The plurality of LEDs 102 populatingboard 104 are operable to produce light with a particular spectral powerdistribution. The color aspect of this spectral power distribution maybe characterized by its centroid wavelength. A centroid wavelength isthe wavelength at which half of the area of the spectral powerdistribution is based on contributions from wavelengths less than thecentroid wavelength and the other half of the area of the spectral powerdistribution is based on contributions from wavelengths greater than thecentroid wavelength. For a plurality of boards, a standard deviation ofthe centroid wavelength can be calculated. In some production examples astandard deviation of the centroid wavelength of a plurality of boardsmay be less than 0.1 nm, e.g., where the boards are populated with LEDscarefully selected for their closely matching spectral powerdistribution or LEDs from a small bin. Of course, costs increasesignificantly when producing boards with a standard deviation of thecentroid wavelength of approximately 0.1 nm or less. In other examples,a standard deviation of the centroid wavelength of a plurality of boardsmay be less than 0.5 nm. In yet other examples, a standard deviation ofthe centroid wavelength of a plurality of boards may be less than 2.0nm.

The LED based illumination device 100 can accommodate LEDs with a widespectral power distribution while still achieving a target color pointwithin a predetermined tolerance. Moreover, multiple LED devices 100 maybe produced, each with one or more LEDs having different spectral powerdistributions, e.g., a large standard deviation of the centroidwavelength, while still achieving closely matched color points from oneLED based illumination device 100 to the next, and where the matchingcolor points of the LED devices 100 are within a predetermined tolerancefrom a target color point. Thus, less expensive LEDs may be used. Byusing two or more wavelength converting materials, the color point ofthe light emitted by the LED based illumination device 100 may beaccurately controlled. In one aspect, the amounts of the two or morewavelength converting materials may be modified based on a colormeasurement of an assembled LED based illumination device such that themodified LED based illumination device emits light within apredetermined tolerance of a target color point. The amounts of thewavelength converting materials may be modified to produce a desireddegree of departure of Δu′v′ between 0.009 and 0.0035 and smaller ifdesired, such as 0.002.

FIG. 14 is illustrative of a system 350 for automatically tuning thecolor of light emitted from an assembled LED based illumination device100 within a predefined tolerance of a target color point by removingportions of two different wavelength converting materials. Although, asillustrated, system 350 automatically tunes LED based illuminationdevice 100 by removal of wavelength converting material, system 350 mayalso be configured to tune LED based illumination device 100 by additionof wavelength converting materials.

System 350 includes an optical detection system 310, a materialmodification planning tool 320, and a material modification system 330.In the embodiment illustrated in FIG. 14, optical detection system 310includes an integrating sphere 311 and a spectrometer 313. In addition,optical detection system 310 includes an optional camera system 314 toimage the light emitted from the surface of LED based illuminationdevice 100. Optical detection system 310 is configured to measure thecolor of light emitted from an LED based illumination device 100 undertest. Although an integrating sphere 311 and spectrometer 313 may beemployed to measure the color of light emitted from an LED basedillumination device 100, other measurement devices may be contemplated.For example, light emitted from LED based illumination device 100 may befiltered by three color filters, each configured to mimic the CIE colormatching functions. After filtering, light detected by a photometer maybe used to determine the three tristimulus values described withreference to Equation 1. Other exemplary color measurement techniquesmay be contemplated.

Material modification planning tool 320 includes a processor 321 and anamount of processor readable memory 322. In the illustrated example,processor 321 and memory 322 are configured to communicate over a bus323. Memory 322 includes an amount of memory 324 storing instructionsthat, when executed by processor 321, implement material modificationplanning functionality as described herein.

In the illustrated embodiment, material modification system 330 includesa controller 331, a laser light source 332, and a galvo scanner 333.Based on a material modification plan generated by material modificationplanning tool 320, controller 331 controls laser 332 and galvo scanner333 to direct radiation emitted from laser 332 to LED based illuminationdevice 100. The incident radiation ablates a portion of a firstwavelength converting material and a portion of a second wavelengthconverting material such that the modified LED based illumination device100 emits colored light within a predetermined tolerance of a targetcolor point. In addition to the illustrated embodiment, other materialmodification systems 330 may be contemplated. For example, a laser basedablation system may employ a variety of motion control schemes toprecisely direct laser light onto LED based illumination device 100. Forexample, a motion system may be used to move the LED based illuminationdevice in one direction and the laser in an orthogonal direction in acoordinated manner. Such a motion system may be employed to preciselydirect laser light alone or in combination with a scanning minor system.In some other examples, material modification system 330 may be amechanical scribing system that mechanically removes material from LEDbased illumination device. Material modification systems 330 based onion etching, chemical etching, electrical discharge machining, plasmaetching, and chemical mechanical polishing may also be contemplated.

In some other examples, material modification system 330 may addmaterial to LED based illumination device 100 to precisely modify theamounts of two different wavelength converting materials. By way ofexample, jet dispensing, spray coating, screen printing, and bladecoating may be employed to precisely add at least two differentwavelength converting materials to LED based illumination device 100 totune the color of light emitted from LED based illumination device 100within a predetermined tolerance of a target color point.

FIG. 16 illustrates a method 300 of automatically tuning the color oflight emitted from an assembled LED based illumination device 100 withina predefined tolerance of a target color point by modifying portions ofat least two different wavelength converting materials. For illustrativepurposes, method 300 is described with reference to system 350illustrated in FIG. 14. However, the execution of the elements of method300 is not limited to the specific embodiments described with referenceto FIG. 14.

In block 301, the color of light emitted by LED based illuminationdevice 100 is measured. LED based illumination device 100 includes twodifferent wavelength converting materials. As illustrated in FIG. 14, anelectrical power source (e.g., current source 312) supplies electricalpower (e.g., current 315) to LED based illumination device 100. Inresponse LED based illumination device 100 emits light having a firstcolor. The emitted light is collected by integrating sphere 311. Bycollecting the light in integrating sphere 311, the sample of lightcollected by spectrometer 313 effectively represents an averaged colorof light emitted from LED based illumination device 100. Spectrometer313 is configured to determine the color of light emitted by LED basedillumination device 100 and communicate a signal 316 indicative of themeasured color to material modification planning tool 320.

In block 302, material modification planning tool 320 determines amaterial modification plan that includes a modification of the amount ofa first wavelength converting material and a modification of the amountof the second wavelength converting material. As modified, the LED basedillumination device will emit light with a changed color point that iswithin a predetermined tolerance of a target color point.

The color point shifts associated with each of the wavelength convertingmaterials is illustrated in the CIE 1931 chromaticity diagram of FIG.17. The color point of the test light source, which produces blue lightat, e.g., 445 nm, is illustrated as point 210 in the diagram. The colorpoint produced by, e.g., wavelength converting material 180 on or withintransmissive plate 174 is illustrated as point 220, which correspondswith a dominant wavelength of, e.g., 630 nm. The color point shiftproduced by wavelength converting material 180 with the test lightsource is along the dotted line 222, where the amount of the shift willdepend on the geometry of the LED based illumination device 100 and thethickness and/or concentration of the wavelength converting material 180on the transmissive plate 174. By way of example, the measured colorpoint produced by wavelength converting material 180 is illustrated bypoint 224 and the shift Δxy from the color point produced by the testlight source without wavelength converting material 180 (e.g., point210) is illustrated by line 226.

The color point produced by, e.g., the wavelength converting material181 on or within transmissive plate 174, is illustrated as point 230which corresponds with a dominant wavelength of, e.g., 570 nm. The colorpoint shift produced by wavelength converting material 181 with the testlight source is along the dotted line 232 depending on the thicknessand/or concentration of the wavelength converting material 181 on thetransmissive plate 174. By way of example, the measured color pointproduced by wavelength converting material 181 with the test lightsource is illustrated by point 234 and the shift Δxy from the colorpoint produced by the test light source without wavelength convertingmaterial 181 (e.g., point 210) is illustrated by line 236. If desired,different formulations of the wavelength converting materials may alsobe used, which would alter the color point produced by the wavelengthconverting materials (as illustrated by arrow 240), and thus, the slopeof the color point shift.

Typically, there is a difference in spectral power distribution from oneLED to the next. For example, LEDs that are supposed to produce bluelight at 452 nm will typically produce light that may range between 450nm and 455 nm or more. In another example, LEDs that are supposed toproduce blue light may produce light that ranges between 440 nm and 475nm. In this example, the spectral power distribution from one LED toanother may be as much as eight percent. The variation in the spectralpower distribution of LEDs is one of the reasons why producing LED basedlight sources with consistent and accurate color points is difficult.However, because the LED based illumination device 100 includes two ormore wavelength converting components with wavelength convertingmaterials that can be individually modified, appropriate wavelengthconverting characteristics can be tuned for a large variation ofspectral power distributions of LEDs 102 to produce a color point thatis within a predetermined tolerance, e.g., a Δu′v′ of less than 0.0035,from a target color point. The target color point may be, e.g., a CCT of2700K, 3000K, 4000K, or other temperature on the black-body curve, oralternatively, the target color point may be off of the black-bodycurve.

FIG. 18 is a diagram illustrating color points of LED devices andpredetermined target color points on the black-body curve from the CIE1960 UCS diagram where the horizontal axis represents CCT and thevertical axis represents the degree of departure (Δuv) from theblack-body curve 400. The target color points may be, e.g., 4000K, 3000Kand 2700K on the black-body curve 400. Other target CCTs or color pointsoff of the black-body curve 400 may be used if desired. FIG. 18illustrates a predetermined tolerance for each of the target colorpoints with a rectangle. For example, at the target color point at 4000Kthe CCT may vary by ±90K, while at 3000K the CCT may vary by ±55K, andat 2700K the CCT may vary by ±50K. These predefined tolerances for CCTare within a two step MacAdam ellipse centered on each respective targetcolor point on the black-body curve. The predetermined tolerance for thedeparture from the black-body curve Δuv for each CCT is ±0.001. In thisexample, Δuv may vary by a distance of 0.001 above the black-body curve400 (expressed as a positive tolerance value, +0.001) and may vary by adistance of 0.001 below the black-body curve 400 (expressed as anegative tolerance value, −0.001). This predetermined tolerance for Δuvis within a one step MacAdam ellipse centered on each respective targetcolor point on the black-body curve. The predetermined tolerances forCCT and Δuv illustrated in FIG. 18 is within a two step MacAdam ellipseand also within the tolerance of Δu′v′ of 0.0035. The color pointswithin the illustrated tolerance from the target color points are soclose that the color difference is indistinguishable for most peopleeven when the light sources are viewed side by side.

The diagram illustrates two color lines centered on the 3000K CCT forreference purposes. One color line 402 corresponds to the color pointshift produced by a first wavelength converting material. In the presentexample, color line 402 is a yellow phosphor coating on the transmissiveplate 174. Color line 404 corresponds to the color point shift producedby a second wavelength converting material. In the present example,color line 404 is a red phosphor coating on the transmissive plate 174.Color line 402 indicates the direction of a shift in color point oflight produced by the yellow phosphor. Color line 404 indicates thedirection of shift in color point produced by the red phosphor. Thefirst wavelength converting material and the second wavelengthconverting material are selected such that their respective directionsof shift in color point are not parallel. Because the direction of shiftof the yellow phosphor and the red phosphor are not parallel, thedirection of the color point shift of light emitted by LED basedillumination device 100 can be arbitrarily designated. This may beachieved by modifying the amount of each phosphor as discussed above. Byway of example, the small spots, 412, 414, 416, and 418 graphicallyillustrate the color points produced by one LED based illuminationdevice 100 using different amounts of wavelength converting materials.For example, spot 412 illustrates the color point for the LED basedillumination device 100 with one set of amounts of the two differentwavelength converting materials. By modifying the amount of yellowphosphor, the color point shifted for the LED based illumination device100 to spot 414. As can be seen, the difference in the color points fromspot 412 to 414 is parallel with the color line 402. By modifying theamount of red phosphor, the color shifts from spot 414 to spot 416 whichis parallel with the color line 404. While this is within the 3000Ktarget, an additional modification of the amount of yellow phosphorresults in a color point illustrated by spot 418, where the shiftbetween spot 416 and 418 is parallel with the color line 402. By againmodifying the amount of yellow phosphor the color point of the LED basedillumination device 100 shifts along line 402 to produce a color pointillustrated by large spot 420, which is well within the predeterminedtolerance from the target color point of 3,000K on the black-body curve.

Material modification planning tool 320 determines the modification ofeach amount of wavelength converting material necessary to shift thecolor of light emitted from LED based illumination device 100 from thevalue measured in block 301 to a target color point within apredetermined tolerance. The modification of each amount of wavelengthconverting material is based on the direction of color shift associatedwith each wavelength converting material and the magnitude of colorshift associated with different amounts of each wavelength convertingmaterial. Material modification planning tool 320 communicates a signal325 indicative of the material modification plan to materialmodification system 330. The material modification plan includes theamount of each wavelength material to be modified and the location onLED based illumination device 100 where each wavelength convertingmaterial should be modified.

In block 303, material modification system 330 modifies the amounts ofthe first and second wavelength converting materials in accordance withthe material modification plan. For example, as illustrated in FIG. 14,controller 331 receives signal 325 indicative of the materialmodification plan. In response, controller 331 controls the laser poweroutput of laser 332 and galvo scanner 333 to remove a portion of atleast two different wavelength converting materials in accordance withthe material modification plan.

FIG. 15 is illustrative of system 350 in another embodiment. In theillustrated embodiment, the optical detection system 310 and thematerial modification system 330 are implemented on a common mechanicalplatform. In this manner, LED based illumination device 100 does nothave to be transported to separate process stations for colormeasurement and material modification.

To tune the color point of light emitted from an LED based illuminationdevice 100, material modification planning tool 320 determines theappropriate modification to each amount of wavelength convertingmaterial necessary to achieve the desired color shift. In addition,material modification planning tool 320 also determines where thematerial modification should occur. In some examples, a thin line or setof lines of wavelength converting material may be added or removed inspecific locations of LED based illumination device 100. In some otherexamples, a series of dots of wavelength converting material may beadded or removed in specific locations of LED based illumination device100.

In another aspect, material modification planning tool 320 determineswhere material modification should occur based on another performancemetric of LED based illumination device 100, in addition to color point.

In one example, the location of a material modification is based atleast in part on an output beam intensity distribution of the LED basedillumination device. FIG. 19 is illustrative of an image 360 collectedby a camera (e.g., camera 314) of the light emitting surface of LEDbased illumination device 100. In one example, this image information317 is communicated to material modification planning tool 320 asillustrated in FIGS. 14 and 15. FIG. 20 is a plot 370 illustrative ofthe luminance 371 across the emitting surface of LED based illuminationdevice 100 at Section line A illustrated in FIG. 19. As highlighted inFIG. 20, the luminance at the emitting surface of LED based illuminationdevice 100 is not perfectly symmetric. For example, portion 372highlighted in FIG. 20 exhibits greater luminance than a correspondingportion opposite axis 373. Based on this measurement, materialmodification planning tool 320 determines a material modification planthat adds wavelength converting materials needed to reach the targetcolor point in the area of portion 372 such that the luminance of themodified device is exhibits improved output beam uniformity.

In another example, the location of a material modification is based atleast in part on achieving an improved color conversion efficiency ofthe assembled LED based illumination device. FIG. 21 shows anillustrative plotline 375 indicative of a spatial variation in intensityof light emitted from LEDs 102 in a plane that is coplanar withtransmissive plate 174. As a result of this spatial variation,wavelength converting material located on transmissive plate 174 issubjected to different levels of excitation light depending on location.For example, in areas of peak intensity the wavelength convertingmaterial may be more sensitive to changes in the amount of materialcompared to areas with less intensity. Thus, changes in the amount ofwavelength converting material may have differing effect on color pointand overall conversion efficiency based on their location. Based on theknown emission pattern and the initial layout of the wavelengthconverting materials, material modification planning tool 320 determinesa material modification plan that adds or removes wavelength convertingmaterials needed to reach the target color point in areas that exhibitimproved color conversion efficiency. For example in FIG. 22, a materialmodification plan includes trajectories of material removal 380A-D thatare a fixed distance from each underlying LED location.

In yet another example, the location of a material modification is basedat least in part on achieving an improved temperature distribution overa light emitting surface of the assembled LED based illumination device.An infrared image of the emission surface of transmissive plate 174 maybe used to determine “hot spots” on the emission surface. These “hotspots” may indicate a disproportionate amount of color conversion. Inresponse, material modification planning tool 320 determines a materialmodification plan that adds or removes wavelength converting materialsneeded to reach the target color point in areas that minimize “hotspots” on the emitting surface of LED based illumination device 100.

In yet another example, the location of a material modification is basedat least in part on achieving an improved color uniformity over a lightemitting surface of the assembled LED based illumination device. Animage of the emission surface of transmissive plate 174 may be used todetermine the color temperature at different locations on the emissionsurface. Differences in color temperature may indicate non-uniformity ofmaterial coatings or differences in peak emission wavelength ofdifferent LEDs 102. In response, material modification planning tool 320determines a material modification plan to add or remove wavelengthconverting materials to reach the target average color point and alsoimproves color temperature uniformity on the light emitting surface ofLED based illumination device 100.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. For example, although LED based illumination device 100is depicted as emitting from the top of the device (i.e., the sideopposite the LED mounting board 104), in some other embodiments, LEDbased illumination device 100 may emit light from the side of the device(i.e., a side adjacent to the LED mounting board 104). In anotherexample, any component of LED based illumination device 100 may bepatterned with phosphor. Both the pattern itself and the phosphorcomposition may vary. In one embodiment, the illumination device mayinclude different types of phosphors that are located at different areasof LED based illumination device 100. For example, a red phosphor may belocated on the bottom side of transmissive plate 174 and yellow andgreen phosphors may be located on the top of transmissive plate 174. Inone embodiment, different types of phosphors, e.g., red and green, maybe located on different areas on transmissive plate 174 or shaped lens172. For example, one type of phosphor may be patterned on transmissiveplate 174 or shaped lens 172 at a first area, e.g., in stripes, spots,or other patterns, while another type of phosphor is located on adifferent second area of on transmissive plate 174 or shaped lens 172.If desired, additional phosphors may be used and located in differentareas. Additionally, if desired, only a single type of wavelengthconverting material may be used and patterned on transmissive plate 174or shaped lens 172. In another example, LED based illumination device100 is depicted in FIGS. 1-3 as a part of a luminaire 150. Asillustrated in FIG. 3, LED based illumination device 100 may be a partof a replacement lamp or retrofit lamp. But, in another embodiment, LEDbased illumination device 100 may be shaped as a replacement lamp orretrofit lamp and be considered as such. Accordingly, variousmodifications, adaptations, and combinations of various features of thedescribed embodiments can be practiced without departing from the scopeof the invention as set forth in the claims.

What is claimed is:
 1. A method comprising: measuring a first color of light emitted from an assembled LED based illumination device that includes an amount of a first wavelength converting material and an amount of a second wavelength converting material; determining a material modification plan based at least in part on the first color of light that includes a modification of the amount of the first wavelength converting material and a modification of the amount of the second wavelength converting material, wherein the assembled LED based illumination device, modified in accordance with the material modification plan, emits a second color of light within a predetermined tolerance of a target color point; and modifying the amount of the first wavelength converting material and the amount of the second wavelength converting material of the assembled LED based illumination device in accordance with the material modification plan.
 2. The method of claim 1, wherein the modification of the amount of the first wavelength converting material includes either adding or removing a portion of the first wavelength converting material.
 3. The method of claim 2, wherein the material modification plan includes a location of the portion of the first wavelength converting material to be modified.
 4. The method of claim 3, further comprising determining the location of the portion of the first wavelength converting material based at least in part on any of an output beam intensity distribution of the assembled LED based illumination device, a color conversion efficiency of the assembled LED based illumination device, a color uniformity of the assembled LED based illumination device, and a temperature distribution over a light emitting surface of the assembled LED based illumination device.
 5. The method of claim 1, wherein the amount of the first wavelength converting material and the amount of the second wavelength converting material are located on a color conversion element of the assembled LED based illumination device.
 6. The method of claim 5, wherein the amount of the first wavelength converting material and the amount of the second wavelength converting material are physically separated on the color conversion element of the assembled LED based illumination device.
 7. The method of claim 5, wherein the color conversion element is an output window of the assembled LED based illumination device.
 8. The method of claim 1, wherein the modification of the amount of the first wavelength converting material produces a color point shift along a first direction in a CIE 1976 u′v′ diagram in response to an amount of light produced by at least one light emitting diode of the assembled LED based illumination device and the modification of the amount of the second wavelength converting material produces a color point shift along a second direction in the CIE 1976 u′v′ diagram in response to the amount of light produced by the at least one light emitting diode, wherein the first direction and the second direction are not parallel.
 9. The method of claim 1, wherein the second color of light has a color point that is within a degree of departure Δu′v′ of 0.009 from the target color point in a CIE 1976 u′v′ diagram.
 10. The method of claim 2, wherein the removing an amount of the first wavelength converting material is achieved by any of laser ablation, mechanical scribing, ion etching, chemical etching, electrical discharge machining, plasma etching, and chemical mechanical polishing.
 11. The method of claim 2, wherein the adding an amount of the first wavelength converting material is achieved by any of jet dispensing, spray coating, screen printing, and blade coating.
 12. The method of claim 1, wherein the amount of the first wavelength converting material is arranged as a first plurality of pixels, and wherein the amount of the second wavelength converting material is arranged as a second plurality of pixels.
 13. The method of claim 12, wherein each of the first plurality of pixels is physically separated from each of the second plurality of pixels.
 14. A method of tuning an assembled LED based illumination device comprising: providing the assembled LED based illumination device including an amount of a first wavelength converting material at a first location and an amount of a second wavelength converting material at a second location; measuring a color of light emitted from the assembled LED based illumination device; determining a modification of the amount of the first wavelength converting material to change the color of light emitted from the assembled LED based illumination device to a target color point within a predetermined tolerance; and selectively modifying the amount of the first wavelength converting material of the assembled LED based illumination device based on the measurement of the color of light emitted from the assembled LED based illumination device.
 15. The method of claim 14, further comprising: determining a modification of the amount of the second wavelength converting material to change the color of light emitted from the assembled LED based illumination device to the target color point; and selectively modifying the amount of the second wavelength converting material based on the measurement of the color of light emitted from the assembled LED based illumination device.
 16. The method of claim 14, further comprising: determining a second color point of light emitted from the assembled LED based illumination device after the modification of the amount of the first wavelength converting material.
 17. The method of claim 16, wherein the color of light emitted from the assembled LED based illumination after modification has a color point that is within a degree of departure Δu′v′ of 0.009 from the target color point in a CIE 1976 u′v′ diagram.
 18. The method of claim 14, wherein the first wavelength converting material produces a color point shift along a first direction in a CIE 1976 u′v′ diagram in response to an amount of light produced by at least one light emitting diode of the assembled LED based illumination device and the second wavelength converting material produces a color point shift along a second direction in the CIE 1976 u′v′ diagram in response to the amount of light produced by the at least one light emitting diode, wherein the first direction and the second direction are not parallel.
 19. The method of claim 14, wherein the first wavelength converting material and the second wavelength converting material are located on a surface of a transparent element.
 20. The method of claim 19, wherein the transparent element is located above at least one light emitting diode of the assembled LED based illumination device.
 21. The method of claim 14, wherein the modifying of the amount of the first wavelength converting material includes any of adding an amount of the first wavelength converting material and removing an amount of the first wavelength converting material.
 22. The method of claim 21, wherein the removing an amount of the first wavelength converting material is achieved by any of laser ablation, mechanical scribing, ion etching, chemical etching, electrical discharge machining, plasma etching, and chemical mechanical polishing.
 23. The method of claim 21, wherein the adding an amount of the first wavelength converting material is achieved by any of jet dispensing, spray coating, screen printing, and blade coating.
 24. The method of claim 14, further comprising: determining a location where the modifying of the amount of the first wavelength converting material should occur, wherein the determining of the location of the modifying of the amount of the first wavelength converting material is based on any of an output beam intensity distribution of the assembled LED based illumination device, a color conversion efficiency of the assembled LED based illumination device, a color uniformity of the assembled LED based illumination device, and a temperature distribution over a light emitting surface of LED based illumination device.
 25. A non-transitory, computer-readable medium, storing instructions that when read by a processor, cause the processor to: receive an indication of a first light having a first color emitted from an LED based illumination device that includes an amount of a first wavelength converting material and an amount of a second wavelength converting material; and generate a material modification plan including a modification of the amount of the first wavelength converting material and a modification of an amount of the second wavelength converting material of the LED based illumination device such that the LED based illumination device emits a second light having a second color within a predetermined tolerance of a target color point after the amount of the first wavelength converting material and the amount of the second wavelength converting material are modified in accordance with the material modification plan.
 26. The non-transitory, computer-readable medium of claim 25, further comprising: receiving an indication of the second color; and determining that the second color is within a degree of departure Δu′v′ of 0.009 from the target color point in a CIE 1976 u′v′ diagram.
 27. The non-transitory, computer-readable medium of claim 25, wherein the material modification plan includes a location of the modification of the amount of the first wavelength converting material and a location of the modification of the amount of the second wavelength converting material.
 28. The non-transitory, computer-readable medium of claim 27, wherein the location of the first wavelength converting material is based at least in part on any of an output beam intensity distribution of the LED based illumination device, a color conversion efficiency of the LED based illumination device, and a temperature distribution over a light emitting surface of the LED based illumination device.
 29. The non-transitory, computer-readable medium of claim 25, wherein the modification of the amount of the first wavelength converting material includes either adding or removing a portion of the first wavelength converting material.
 30. The non-transitory, computer-readable medium of claim 25, wherein the modification of the amount of the first wavelength converting material produces a color point shift along a first direction in a CIE 1976 u′v′ diagram in response to an amount of light produced by at least one light emitting diode of the LED based illumination device and the modification of the amount of the second wavelength converting material produces a color point shift along a second direction in the CIE 1976 u′v′ diagram in response to the amount of light produced by the at least one light emitting diode, wherein the first direction and the second direction are not parallel. 